.main{
    width: 100%;
    height: 50%;
    position: relative;
    background: -webkit-linear-gradient(yellow, red); /* Safari 5.1 - 6.0 */
    background: -o-linear-gradient(yellow, red); /* Opera 11.1 - 12.0 */
    background: -moz-linear-gradient(yellow, red); /* Firefox 3.6 - 15 */
    background: linear-gradient(yellow, red); /* 标准的语法 */
}
.main-box{
    height: 100px;
    line-height: 100px;
    text-align: center;
    margin: auto;
    position: absolute;
    top: 0; left: 0; bottom: 0; right: 0;
}
#btn{
    width: 100px;
    height: 60px;
    margin: 100px auto;
    display: block;
}
#redball{
    color: #fff;
}
#blueball{
    color: blue;
}